Wireless communication devices typified by mobile phone terminals spread rapidly with the developments in mobile communication systems. For example, in the mobile phone terminals, there has been used high frequency bands such as 800 MHz to 1.0 GHz and 1.5 GHz to 2.0 GHz. Duplexers using a high-frequency filter formed by combining resonators are used in the aforementioned communication devices. As the resonator, used are surface acoustic wave resonators and piezoelectric thin film resonators, for example.
As the wireless communication devices are desired to be reduced in size, there has been developed technology that makes it possible to reduce the size and height of the duplexer as disclosed in Japanese Patent Application Publication No. 2008-271230. In addition, to improve noise characteristics, the use of a balanced-mixer and a balanced low noise amplifier (LNA) has been promoted in a receive circuit, and the use of a balanced-type receive filter has been promoted in the duplexer. From this kind of circumstance, there has been developed technology to improve balance characteristics of the balanced-type filter as disclosed in Japanese Patent Application Publication No. 2005-318307.
In addition, as a high-frequency filter used in the duplexer, there has been ladder-type filters in which a series resonator and a parallel resonator are connected in a ladder form. There has been developed technology that makes it possible to improve characteristics of the ladder-type filter and reduce the size of the ladder-type filter as disclosed in Japanese Patent Application Publication Nos. 2009-207116 and 2007-74698.
When the ladder-type filter is used for a transmit filter connected between an antenna terminal and a transmit terminal, unnecessary waves increase and second harmonic characteristics degrade.